There is an increasing demand for the use of microstrip antennas in wireless communication due to their inherently low back radiation, ease of conformity and high gain as compared to wire antennas. The microstrip antenna design allows for a small amount of radiation produced in one direction, the back of the antenna. The low back radiation generated from a microstrip antenna is important in shielding the human user of the transmitting instrument from the possibly hazardous electromagnetic fields caused during transmissions. A desired application of microstrip antennas is for use in a cellular phone system.
FIG. 1 shows a conventional microstrip antenna for receiving communication signals. The antenna 100 includes a dielectric substrate 101 mounted on a large metallic ground plate 103 with a resonant metallic patch 105 (radiating element) affixed to the opposite side of the substrate 101. The dimensions of the resonant patch is selected as a function of the wavelength of the signals the antenna is to receive and transmit. The length of one side of a square radiating element must be .lambda./2, where .lambda. is the wavelength of the transmission signals. Thus, if a wavelength of a cellular signal which is to be received by a microstrip antenna is approximately 36 cm, then the dimension of one side of the microstrip antenna must be 18 cm. The antenna's required dimension is larger than the conventional cellular phone's width and therefore unusable for a cellular phone application.
The dimensions of the conventional microstrip antenna can be reduced by increasing the dielectric constant of the microstrip substrate. However, a correspondingly thicker dielectric material is very expensive and is lossy for signal transmissions. Additionally, wireless communication requires a bandwidth of more than 3% which is normally the upper limit of the conventional microstrip antenna's bandwidth. By increasing the dielectric constant of the substrate to reduce the size of the antenna, the bandwidth of the antenna will also be reduced significantly. Thus, microstrip antennas with high dielectric constants cannot meet the common bandwidth specifications of wireless systems.
The low electromagnetic (EM) radiation of a conventional microstrip antenna is useful only in protecting one portion of the cellular phone user, the user's head, which is in the direction of the ground plate. There is no direct path of grounding for the input signal existing on the radiating plate. As a result, strong induced currents exist on the radiating plate. These currents can leak to the user's hand and body which come in contact with the cellular phone.
It would be advantageous to have a microstrip antenna which would limit radiation and surface currents, to be small enough to be practical in a cellular phone or other communication instrument, to have sufficient bandwidth to allow proper operation in wireless communication and to have an improved gain necessary for communication.